1. Field of the Invention
The present invention relates to an information processing apparatus, an information processing system, an information processing method, a computer readable storage medium storing a program for realizing the information processing method, and the program, all of which are used with the apparatus or system for detecting falsification of digital data in accordance with a human sense, the digital data including, for example, image data containing moving images and still images and voice data such as music.
2. Description of the Related Art
“Digital data” is intended to mean digitalized information, and “digital contents” are intended to mean the contents or meaning of digital data of a still image such as photographs, a moving image such as movie, or voices such as music.
Digital data is easy to be subjected to processes such as editing and compression processes. It is therefore easy to falsify digital data. It is an important issue to detect the presence/absence of falsification of digital data in order to protect its copyright.
There is a method of guaranteeing the originality of digital data by calculating a hash value of the digital data and executing a digital signature by using public key cryptography. For example, in the system disclosed in U.S. Pat. No. 5,499,294, a transmission side of digital data has a secret key Ks and a public key Kp. The transmission side of the digital data executes a calculation process of compressing plaintext data M by a hash function and outputting an output h having a predetermined length.
The hash function has a function of outputting data h having a predetermined length by calculating plaintext data M having an arbitrary length. The output data h is called a hash value of plaintext data M.
The hash function is a one-way function having a difficulty, in terms of calculation amount, in calculating plaintext data M satisfying “h=H(M)” from a given output data h. The hash function also has the collision tolerance that it is difficult in terms of calculation amount to calculate plaintext data M′ satisfying “H(M)=H(M′)” (M is not equal to M′) and it is difficult in terms of calculation amount to calculate plain text data M and M′ satisfying “H(M)=H(M′) and M is not equal to M′)”.
Known hash functions include MD-2, MD-4, MD-5, SHA-1, RIPEMD-128, RIPEMD-160 and the like. The algorithms of these functions are made public.
Next, the transmission side of the digital data executes a calculation process of converting the output h by using the secret key Ks and outputting the conversion result as a digital signature s, namely by using an equation of D(Ks, h)=s.
The transmission side transmits the digital signature s and plaintext data M to the reception side of the digital data.
The reception side of the digital data executes: a calculation process of converting the digital signature s transmitted from the transmission side by using the public key Kp, namely by using the equation E(Kp, s)=E(Kp, D(Ks, h″))=h″; and a calculation process of calculating h′ by compressing the plaintext data M′ transmitted from the transmission side of the digital data by using the same hash function used by the transmission side. The reception side of the digital data judges that the plaintext data M′ is correct if h′ and h″ are coincident.
If the plaintext data M transmitted from the transmission side of the digital data is altered or falsified before the plaintext data M is received at the reception side of the digital data, E(Kp, s)=E(Kp, D(Ks, h″))=h″ is not coincident with h′ obtained by compressing the plaintext data M′ by the hash function so that it is possible to detect falsification of the plaintext data M.
Since the hash function is a one-way function, if the plaintext data M is falsified, the digital signature s is also falsified. Therefore, there is no possibility of an inability to detect falsification of the plaintext data M.
With this method, falsification can be detected. For example, if original digital image data is inserted with other image contents, if original digital image data is partially deleted, or if original digital image data is compressed, the altered digital image data is different from the original digital image data even by one bit.
For example, if digital image data transmitted from a transmission side is compressed by a JPEG compression method and developed, the digital image data becomes quite different from the original digital image data. However, the human sense recognizes it as the same image contents. If digital image data transmitted from a transmission side is superposed with noises such as an electronic watermark unable to be distinguished with human eyes, the digital image data becomes quite different from the original digital image data. However, the human sense recognizes it as the same image contents.
The reason for this may be ascribed to that a high level of redundancy generally exists in image contents. From this reason, the reception side of digital image data recognizes falsification even if the alteration process unable to detect with human sense is performed, i.e., even if the alteration process is performed which does not essentially change the contents of the digital image data.